A substantial body of evidence indicates that donor dendritic cells may play a pivotal role in the initiation of allograft rejection. However, the precise cellular and molecular mechanisms by which DC activate T cells and initiate allograft rejection are poorly understood. Our central hypothesis of how DC trigger the rejection response holds that: 1) dendritic cells exist in non-lymphoid organs (i.e., the allograft) in a non-stimulatory form; 2) the process of transplantation leads to rapid elaboration of the cytokines, GMCSF and IL-1 within the graft; 3) these cytokines cause DC within the graft to mature into potent T cell stimulators by up-regulating the critical costimulatory molecule B7; 4) donor DC then migrate out of the grafts via blood and lymph into the lymphoid tissues, where they activate the recipient's T cells. In this application, we propose to study the expression and function of B7 on dendritic cells and other APC in murine systems in vitro and in vivo, with particular emphasis on its expression during the initiation of allograft rejection. We will also evaluate strategies designed to interfere with B7 expression and/or function in vitro and in vivo. Using flow cytometry, molecular techniques, and functional assays in vitro, and murine cardiac and skin transplant models in vivo:, we propose to: 1) Evaluate changes in the expression of B7 in non-lymphoid dendritic cells during their functional maturation into potent immunostimulatory cells; 2) Define which cytokines regulate expression of B7 on members of the DC lineage and other antigen presenting cells (lymphoid DC, Langerhans cells, cardiac and kidney DC, macrophages and endothelial cells); 3) Study longitudinal changes in B7 expression relative to various cytokines in skin grafts and vascularized cardiac allografts/isografts; 4) Study the ability of strategies which interfere with B7 expression and function on DC to prolong allograft survival (e.g., treatment with CTLA4-Ig, IL-10, or anti-GMCSF). Since the submission of our original proposal there have been several developments which have greatly encouraged us as to the feasibility and importance of the proposed studies. We and others have reported that DC express the critical costimulatory molecule B7 and that blockade of B7 on DC prevents T cell proliferation in MLR. Our studies on Langerhans cells indicate that expression of B7 on DC from non-lymphoid tissues (i.e., those contained within transplanted organs) may be tightly regulated by locally produced cytokines. Other recent evidence indicates that B7 blockade in vivo can result in prolongation or even long-term acceptance of allografts and xenografts. While all of these observations are exciting and encouraging, still relatively little is known of the distribution, regulation of expression, and function of B7 on APC, in particular, dendritic cells. The proposed investigations on the expression of B7 in non-lymphoid tissues, the cytokines and circumstances that up-regulate its expression, as well as further investigation on the efficacy of B7 blockade to induce graft survival in murine systems are likely to yield important new information to assist in development of transplantation tolerance.